LONDON – Stopping treatment with dasatinib and nilotinib might be safe in chronic myeloid leukemia patients who have achieved sustained and complete molecular responses, early data from a pilot study have shown.

The STOP 2GTKI study is a successor to the STop IMatinib (STIM) trial, which showed that stopping treatment with imatinib – the first-generation tyrosine kinase inhibitor (TKI) of BCR-ABL – could be feasible in patients who achieve long-lasting complete molecular responses (Lancet Oncol.2010;11:1029-35).

Although still preliminary, the new data suggest that it could be possible to do the same with these second-generation TKIs in patients who were previously intolerant or resistant to treatment with imatinib, Dr. Delphine Réa said at the annual Congress of the European Hematology Association.

"There are different kinds of reasons why we should aim at stopping tyrosine kinase inhibitors in chronic myeloid leukemia patients," said Dr. Réa of Hôpital Saint-Louis in Paris.

"The first one relates to patients. From the patients’ perspective it’s very important to have the hope of feeling cured," said Dr. Réa, a member of the French team that performed both the STIM and the STOP 2G-TKI studies.

Other reasons given include avoiding the adverse effects of prolonged therapy on quality of life, and reducing health care costs by steering clear of unnecessary long-term treatment for what becomes a chronic disease. It’s also important for clinicians to be able to show that a treatment really does cure a disease, she said, and the only way to do that is to stop the treatment.

Dr. Réa presented early data on 14 patients who had been enrolled in the STOP 2G-TKI pilot study until May 2011 and for whom there was at least 6 months’ follow-up. The median age of patients was 59 years.

"I think it’s very important to have 6 months’ follow-up at least because in the STop IMatinib trial, most of the relapses occurred within the first 6 months," Dr. Réa explained in an interview after her presentation.

"Here we didn’t have any relapses after 6 months," Dr. Réa added. "For the nine patients who didn’t relapse, the median follow-up is 10 months." The duration of time that these patients remained treatment free was between 6 months and 19 months.

For inclusion in the study, patients had to be aged 18 years or older with chronic or acute phase chronic myeloid leukemia at diagnosis, and be receiving ongoing dasatinib or nilotinib treatment after intolerance or resistance to imatinib therapy. TKI treatment had to have been administered for at least 3 years, and patients needed to have undetectable levels of BCR-ABL with at least 20,000 copies of ABL.

The primary end point was the achievement of a stable major molecular response (MMR) at 6 months, determined by monthly blood counts and real-time quantitative polymerase chain reaction for the first year of discontinuation, then every 2-3 months thereafter. Bone marrow smears and cytogenic and mutational analysis are undertaken if there is a rise in BCR-ABL above an internationally standardized (IS) ratio of 1%. Patients are re-treated with dasatinib or nilotinib if there is a loss of MMR of more than 0.1% IS.

Results showed that at 6 months’ follow-up, 71% of patients had a persistent MMR, and 64% remained treatment free. Five patients restarted TKI treatment at a median of 5 months, and four patients lost MMR after a median of 3 months. However, the patients who lost major molecular response retained their sensitivity to TKIs, and BCR-ABL was undetectable in all five patients who restarted therapy.

Asked what these results could mean for clinical practice, Dr. Réa noted that they are an early indicator that stopping treatment with second-generation TKIs will probably be an option for some patients who achieve good responses. Careful (at least monthly) follow-up of patients who cease treatment would be required, at least in the research setting.

"We don’t know, but we hope the relapse rate will be lower than we have seen with imatinib [discontinuation]," Dr. Réa said. She added, however, that there is an indication that the relapse rate could actually be higher than that seen in the STIM trial.

As yet, the optimal duration of treatment with a second-generation TKI before discontinuation can be attempted is unknown. "In the STop IMatinib trial, it seems to be between 4 and 5 years," Dr. Réa observed. "So I don’t think it will be reasonable to stop treatment before that period of time."

Further results from the trial can be expected later in the year, Dr. Réa noted. To date, 23 patients have been recruited and 8 more are to be included in the trial.

Dr. Réa disclosed serving on advisory boards of Bristol-Myers Squibb and Novartis.

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